Technical Contact
Dr. William J. Merline
(303) 546-0487
merline@boulder.swri.edu
Media Contact
Maria Martinez
(210) 522-3305
mmartinez@swri.org
ASTRONOMERS IMAGE DOUBLE ASTEROID
AND A NEW ASTEROID MOON
Embargoed for release until Thursday, October 26, 2000, at 3:00 PM EDT
Boulder, Colorado -- October 26, 2000 -- Large telescopes with
deformable optics are allowing astronomers to study distant asteroids
with unprecedented clarity -- leading to the discovery of new shapes and
configurations and presenting scientists with new puzzles to solve.
An international team of astronomers led by Dr. William Merline of the
Boulder office of Southwest Research Institute (SwRI) released today the
first-ever images of a large, double asteroid. Each asteroid in the pair
is the size of a large city (about 50 miles across), separated by about
100 miles, mutually orbiting the vacant point of interplanetary space
that lies midway between them. The discovery was made using the W.M.
Keck Observatory atop Mauna Kea, the tallest mountain in Hawaii. The
asteroid pair was once assumed to be a single body, called Antiope,
orbiting the sun in the outer parts of the asteroid belt between the
orbits of Mars and Jupiter.
The team also released a picture of a small moon orbiting the large
asteroid Pulcova. This moon was discovered in February 2000 using the
Canada-France-Hawaii Telescope (CFHT), also on Mauna Kea. It is only the
third asteroid discovered to have a small moon. Asteroid-moon pairs had
not been seen until 1993, when the Galileo spacecraft imaged the
one-mile-wide moonlet Dactyl, as it rushed past the 19-mile-diameter
asteroid Ida. The Merline team reported the second asteroidal moonlet a
year ago, circling the 135-mile-sized asteroid Eugenia. The team named
the companion Petit-Prince, officially accepted by the International
Astronomical Union in August.
"It's getting to be kind of bewildering," says Dr. Christophe Dumas of
the Jet Propulsion Laboratory (JPL), a team astronomer. "Asteroids were
once thought to be single, mountain-like chunks of material, perhaps
smashed into 'flying rubble piles' by occasional collisions among
themselves."
Astronomers expect strange new configurations to provide still more
surprises as the survey continues. "Every new asteroidal companion we
discover seems to bring new configurations and new mysteries," says team
member Dr. Clark R. Chapman, also of the SwRI Boulder office.
The team's approach uses a new technology, called adaptive optics, which
enables telescopes to see asteroids and other small points of light in
the heavens with the same clarity as the Hubble Space Telescope. Until
recently, ground-based telescopes were hindered by distortions caused by
Earth's atmosphere, in much the same way water distorts the view of an
underwater object. The new technique passes light from the telescope
through a specialized "correction box" to instantaneously analyze the
distorted light and compute the amount of correction necessary to remove
the blurring of the atmosphere. The correction information is then fed
to deformable mirrors in the box that remove the distortion, providing a
sharper image.
A fascinating demonstration of the new telescope technology is in a
movie of the asteroid Kleopatra, also released today, observed during a
seven-hour period. Earlier this year, Steve Ostro of JPL published
reconstructions of Kleopatra's shape based on radar reflections obtained
when that asteroid was fairly close to the Earth in November 1999.
During the same month, team member Dr. Francois Menard, currently a
visiting scientist at CFHT, obtained adaptive optics images. "Excellent
agreement of both optical and radar pictures of Kleopatra's 'dog-bone'
shape provides added confidence in the reliability of adaptive optics
images," says Menard.
"Radar works well for asteroids near the Earth, but adaptive optics is
much more powerful for studying asteroids in the middle of the asteroid
belt and beyond," says Dr. Laird Close of the European Southern
Observatory and the University of Arizona.
This week, Merline and his colleagues reported to an annual meeting of
international scientists specializing in solar system studies on two
years of asteroid surveys conducted at three observatories equipped with
the new adaptive optics systems.
"In fact, large asteroidal satellites and twin companions are rather
rare," Merline told attendees of the 32nd annual meeting of the American
Astronomical Society's Division for Planetary Sciences, convened this
week in Pasadena, California. "Preliminary study of about 200 asteroids
has turned up only two asteroids with moons (Eugenia and Pulcova) and
just one double (Antiope)," he explains. "It is possible that a few more
moonlets might emerge from more sophisticated analysis of the data we
have collected."
Pulcova is an asteroid about 90 miles in diameter. Its small satellite,
roughly a 10th its size, orbits Pulcova every four days at a distance of
about 500 miles.
Asteroidal companions provide vital information about asteroids that has
been difficult to obtain. Until now, the best measurements of asteroid
masses -- their bulk densities, such as whether they are "light" like
ice, "dense" like metal, or in between like rocks -- came from
deflections of spacecraft flying past an asteroid. Such spacecraft
encounters are rare, and deflections of more distant objects (other
asteroids or planets) by an asteroid's gravity are weak and difficult to
measure. But an asteroidal satellite, or twin, is a body whose
trajectory is so mightily deflected by the asteroid's gravity that it is
actually forced to orbit around it. The revolution time provides a
measure of the body's mass, hence density. Using such techniques,
Merline's team find that Eugenia, Pulcova, and Antiope are all rather
light bodies. They are much less dense than familiar rocks, more like
ice, but their surfaces appear very dark, like rock. Interesting
differences in the densities motivate further research on asteroids with
satellites.
NASA and the National Science Foundation are funding this research.
Observations are being conducted at the Keck Observatory and the CFHT
(operated by the National Research Council of Canada, the French Centre
National de la Recherche Scientifique, and the University of Hawaii).
Other team members are Dr. J. Chris Shelton (Mt. Wilson Observatory) and
Dr. David Slater (SwRI, San Antonio).
EDITORS: Images accompanying this press release will be available at
11:00 AM EDT at http://www.boulder.swri.edu/merline/press.
SwRI is an independent, nonprofit, applied research and development
organization based in San Antonio, Texas, with more than 2,700 employees
and an annual research volume of more than $300 million.